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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 11 October 2011 by dana1981, Albatross

Dr. Roger Pielke Sr. has written a blog post addressing the disagreement between himself and Skeptical Science (SkS) regarding the contribution of CO2 to the net positive anthropogenic radiative forcing. Initially Dr. Pielke cited a presentation he gave in 2006 which said (on slide 12):

"The CO2 contribution to the radiative warming decreases to 26.5% using the IPCC framework given in Slide 9"

This "radiative warming" refers to the human plus natural positive radiative forcings ('natural' being solar). As Dr. Pielke's presentation was given in 2006, before publication of the IPCC Fourth Assessment Report (AR4), his reference to the IPCC is to the Third Assessment Report (TAR) published in 2001. In his new post, Dr. Pielke also references a previous post on his blog on the same subject, which concludes (emphasis added):

"For all of the human-caused warming radiative forcings, which includes the 0.5 Watts per meter squared value for the shortwave albedo change, and estimating tropospheric ozone as 0.3 Watts per meter squared, the aerosol black carbon direct effect as 0.2 Watts per meter squared, the black carbon on snow and ice as 0.3 Watts per meter squared, the semidirect indirect effect as 0.1 Watt per meter squared, and the glaciation indirect effect as 0.1 Watt per meter squared (with the latter two forcings using a nominal value, since these forcings are very poorly known), the contribution due to CO2 will fall to about 28%."

In this case Dr. Pielke refers to only the human positive radiative forcings, excluding the contribution of solar irradiance.

In short, Dr. Pielke has argued that CO2 contribution to the total positive radiative forcing (since pre-industrial times) is between 26% and 28% (depending on whether solar effects are included), whereas in our previous post, SkS concurred with the AR4 radiative forcing estimates, which put CO2 at approximately 50% of the total positive radiative forcing (nearly twice Dr. Pielke's estimate).

Below we discuss some problems SkS has identified in Dr. Pielke's estimate, and provide a detailed up-to-date estimate of these values. The main underlying problem is that Dr. Pielke is relying on an estimate he made in 2006, failing to account for advances in climate research over the past 5 years, and thus his sources are at least 5 years out of date. Additionally, he appears to have made some mathematical errors in his calculations.

Methane

Dr. Pielke estimates the radiative forcing from methane at 0.8 Watts per square meter (W/m2), which is significantly larger than the IPCC estimate (both TAR and AR4) of 0.48 W/m2. To support this value, in his 2006 presentation Dr. Pielke references research by "Drew Shindell and colleagues; Keppler et al." (slide 11), and on his blog posts, references Keppler et al. (2006). Keppler et al. do not estimate the methane radiative forcing in their paper - the 0.8 W/m2 figure is Dr. Pielke's estimate based on Keppler et al.'s results.

"By summing the 0.8 Watts per meter squared for methane and using the total of 2.4 Watts per meter squared of the well-mixed greenhouse gases from the IPCC Report..."

The 0.48 W/m2 methane forcing is included in the 2.43 W/m2 best estimate forcing for well-mixed greenhouse gases in the IPCC TAR (the best estimate is 2.64 W/m2 in the AR4). Thus, summing Pielke's estimated methane forcing (0.8 W/m2) and the IPCC TAR greenhouse gas forcing (2.4 W/m2) double counts the methane forcing.

Albedo

Dr. Pielke also estimates "0.5 Watts per meter squared value for the shortwave albedo change," which is a forcing not included in the TAR or AR4. In his presentation (slide 11), Dr. Pielke claims:

"For the period 2000-2004, a CERES Science Team assessment of the shortwave albedo found a decrease by 0.0015 which corresponds to an extra 0.5 W m−2 of radiative imbalance according to their assessment."

However, there are a number of problems with this estimate. Most importantly, the data in question only cover a period of 4 years. Changes in the Earth's albedo (reflectivity) over a 4-year period tell us little or nothing about changes in albedo over the past century. It's apples and oranges; one is short-term, the other is long-term.

Four years is also simply far too short of a timeframe to ascertain a meaningful trend. From Loeb et al. (2007):

"Commonly used statistical tools applied to the CERES Terra data reveal that in order to detect a statistically significant trend of magnitude 0.3 W m−2 decade−1 in global SW TOA flux, approximately 10 to 15 yr of data are needed. "

Loeb et al. (2007) also used a revised version of the CERES data to show that no statistically significant changes in the Earth’s albedo occurred between 2000 and 2005. More recently, Palle et al. (2009) conclude:

"Earthshine and FD [International Satellite Cloud Climatology Project flux data] analyses show contemporaneous and climatologically significant increases in the Earth's reflectance from the outset of our earthshine measurements beginning in late 1998 roughly until mid-2000. After that and to date, all three show a roughly constant terrestrial albedo, except for the FD data in the most recent years"

We should also note that an albedo increase/decrease due to increasing cloud cover would also be accompanied by an increased/decreased greenhouse effect, making the net effect on the climate even more uncertain.

But the bottom line is that in order to incorporate an albedo forcing into these estimates, we must use an estimated albedo change from pre-industrial to Present. We should also investigate the cause of any albedo change to determine if it should be treated as a forcing or as a feedback. If it's a forcing, then it's not anthropogenic, and Dr. Pielke was incorrect to include it in the anthropogenic forcings. If it's a feedback, then it should not be included in the calculation of total forcings at all.

Ultimately, for this calculation, Dr. Pielke's 0.5 W/m2 albedo forcing estimate is unjustified and not supported by more recent observations and scientific literature.

Black Carbon

Dr. Pielke cites Hansen and Nazarenko (2004) in estimating the albedo effect of soot on snow and ice at 0.3 W/m2, and the net black carbon forcing at 0.5 W/m2. However, as the IPCC AR4 noted three years later, the magnitude of the black carbon radiative forcing remains uncertain. The best estimate of Skeie et al. (2011) of 0.45 W/m2 for the black carbon forcing is in rough agreement with Dr. Pielke's estimate. Ramanathan and Carmichael (2008) give a best estimate for the black carbon forcing at 0.9 W/m2.

Tropospheric Ozone

Dr. Pielke's tropospheric ozone forcing estimate is somewhat unclear. He states that the associated forcing is 0.3 W/m2, but the IPCC TAR estimate is 0.35 W/m2, and Dr. Pielke appears to believe the value should be higher:

"Ozone was responsible for one-third to one-half of the observed warming trend in the Arctic during winter and spring [Drew Shindell]"

This release from NASA GISS appears to be the source, from which, if we are interpreting his presentation correctly, Dr. Pielke estimates an additional 0.3 W/m2 on top of the IPCC 0.35 W/m2 tropospheric ozone radiative forcing.

However, there are more recent estimates of this forcing, in addition to the IPCC's 0.35 W/m2 (both TAR and AR4). The best estimate from Skeie et al. (2011) was 0.44 W/m2, and the best estimate from Cionni et al, 2011 (submitted), on which Shindell is a co-author, is 0.23 W/m2. Thus Dr. Pielke's estimate of 0.65 W/m2 appears to be much too high.

Aerosol Semi-Direct and Indirect Effects

Dr. Pielke also identifies a "glaciation effect" as causing a 0.1 W/m2 forcing, which, in a recent talk, he clarifies as "An increase in ice nuclei increases the precipitation efficiency." Lohmann et al. (2007) is a very good paper on this subject, and explains the effect, described as the aerosol indirect effect:

"Global climate model studies suggest that if, in addition to mineral dust, hydrophilic black carbon aerosols are assumed to act as ice nuclei at temperatures between 0 and –35°C, then increases in aerosol concentration from pre-industrial to present times may cause a glaciation indirect effect (Lohmann, 2002a). The glaciation effect refers to an increase in ice nuclei that results in a more frequent glaciation of supercooled stratiform clouds and increases the amount of precipitation via the ice phase. This decreases the global mean cloud cover and allows more solar radiation to be absorbed in the atmosphere. Whether or not the glaciation effect can partly offset the warm indirect aerosol effect depends on the competition between the ice nucleating abilities of the natural and anthropogenic freezing nuclei (Lohmann and Diehl, 2006)."

"Despite the high complexity and nonlinearity of the microphysical interaction between aerosols and clouds, modeling studies generally indicate that the net effect of this interaction is to reflect more radiation back to outer space [Forster et al., 2007], although recent results show that aerosols acting as ice nuclei could counteract the cooling effect significantly [Storelvmo et al., 2008]. A few observational studies seem to confirm a relation between soil dust aerosols and cloud cover."

In short, the aerosol indirect glaciation effect remains far from clear. Dr. Pielke also identifies the aerosol semi-direct effect, which involves tropospheric aerosols absorbing shortwave radiation, as causing a 0.1 W/m2 forcing. However, the IPCC has not included this as a positive forcing becase

"the semi-direct effect is not strictly considered an RF because of modifications to the hydrological cycle"

"The semi-direct effect refers to temperature changes due to absorbing aerosols that can cause evaporation of cloud droplets, as was shown in a large eddy model simulation study that used black carbon concentrations measured during the Indian Ocean Experiment (Ackerman et al., 2000). It ranges from 0.1 to –0.5 Wm-2 in global simulations"

The IPCC AR4 also lists the semi-direct effect as "positive or negative" and "small" potential magnitude, and the indirect effect as "positive" and "medium" potential magnitude, where as Dr. Pielke lists both as positive and equal in magnitude (0.1 W/m2). In short, the magnitude and roles of the aerosol semi-direct and indirect glaciation effects in terms of radiative forcings remain far from clear.

Carbon Dioxide

Dr. Pielke's estimate for the CO2 radiative forcing (1.4 W/m2) is both outdated and not consistent with the value in the IPCC TAR (1.46 W/m2); it appears that he either rounded the value down or eyeballed the IPCC TAR radiative forcing graphic rather than looking up the precise value. However, 1.46 W/m2 was the estimated value in 2001, when the TAR was published. In 2007, when the AR4 was published, the CO2 forcing had already increased to 1.66 W/m2. More recently, the NOAA AGGI estimated the CO2 forcing at 1.79 W/m2 in 2010, and Skeie et al. at 1.82 W/m2.

In other words, the CO2 radiative forcing has increased 25% over the past decade. Some of the other forcing estimates (like tropospheric ozone and black carbon) have changed mainly as a result of new research, but the CO2 forcing has changed as a result of rapidly increasing CO2 emissions and atmospheric concentrations.

"I think it is very generally recognized that, for the same global mean forcing, aerosols perturb the mean precipitation field more than do the well-mixed greenhouse gases (WMGGs). So if, up to the present, anthropogenic aerosols and WMGGs have had comparable effects on regional precipitation, say, the WMGG effect will undoubtedly grow and will be essentially irreversible on the time scale of several centuries, in the absence of geoengineering, while the aerosol effect will likely be bounded by its current magnitude, and the WMGGs will dominate."

Estimated CO2 Contribution

Below we summarize various estimates of the CO2 contribution to the net positive radiative forcing. We believe Dr. Pielke has committed two types of errors: mathematical (double-counting and rounding), and using outdated sources.

We believe the first column is a replication of Dr. Pielke's estimates. The second column corrects Dr. Pielke's math errors by eliminating the double counting of methane, and correcting rounding errors for the CO2 and solar forcings. The third column provides the IPCC TAR estimates which were the basis of Dr. Pielke's estimates, but which, for the most part, we believe are more accurate than Dr. Pielke's suggested values.

The fourth and fifth columns correct for the out-of-date references by using the IPCC AR4 and Skeie et al. (2011) estimates. Bear in mind we have not included the uncertainty ranges - these are all just best estimates of the respective positive radiative forcings (in W/m2).

Forcings

Pielke 2006

Pielke Math Corrected

IPCC TAR

IPCC AR4

Skeie 2011

CO2

1.40

1.46

1.46

1.66

1.82

CH4

0.80

0.80

0.48

0.48

0.49

other LLGHGs

1.00

0.49

0.49

0.50

0.51

tropospheric ozone

0.65

0.65

0.35

0.35

0.44

black carbon

0.50

0.50

0.20

0.10

0.45

albedo

0.50

0.50

0

0

0

aerosols (semi-direct+indirect)

0.20

0.20

0

0

0

stratospheric water vapor

0

0

0

0.07

0.07

contrails

0.02

0.02

0.02

0.01

0

solar

0.25

0.30

0.30

0.12

0.12 (AR4)

Total Positive Forcing

5.32

4.92

3.30

3.29

3.90

Total Anthropogenic Forcing

5.07

4.62

3.00

3.17

3.78

CO2 contribution to Total

26.3%

29.7%

44.2%

50.5%

46.7%

CO2 contribution to Anthropogenic

27.6%

31.6%

48.7%

52.4%

48.2%

If we correct for Pielke's double counting and rounding errors, the CO2 contribution to the total net positive forcing increases to approximately 30%. When we use up-to-date research for all forcings, the CO2 contribution increases to close to 50%, as we originally argued. We again note that this fraction will continue to increase along with continually increasing human CO2 emissions.

Human Contribution to Global Surface Warming

"Approximately what percentage of the global warming (increase in surface, atmosphere, ocean temperatures, etc.) over the past 100 years would you estimate is due to human greenhouse gas emissions and other anthropogenic effects?"

We suggest a back-of-the-envelope answer to this question by applying the probabilistic estimate of transient climate sensitivity by Padilla (2011):

"we find a most-likely present-day estimate of the transient climate sensitivity to be 1.6 K with 90% confidence the response will fall between 1.3–2.6 K"

We can use this range of transient climate sensitivity (alpha = 0.35 to 0.70 K/Wm-2) and scale the transient climate response (we're currently 49% of the way to the radiative forcing associated with CO2 doubling [~1.8 out of 3.7 W/m2]) to estimate the amount of CO2-caused surface warming:

Where F is the radiative forcing. Using the Skeie et al. (2011) CO2 forcing best estimate of 1.82 W/m2 for 2010 and the Padilla (2011) range of transient climate sensitivity parameters, this corresponds to a CO2 contribution of 0.64 to 1.28°C, with a best estimate of 0.79°C warming of average global surface temperature.

We can also consider the expected warming for the net anthropogenic forcing, which Skeie et al. estimated at 1.4 W/m2 and the IPCC AR4 estimated it at 1.6 W/m2. Using these two estimates and the Padilla transient sensitivity range yields a net anthropogenic warming of 0.49 to 1.12°C with a central estimate of 0.65°C warming of average global surface temperature.

Dr. Pielke, would you concur with these estimated ranges of CO2 and anthropogenic warming?

Take-Home Message

The main points here are that CO2 is responsible for approximately 50% of the net positive radiative forcing since pre-industrial times (a percentage which will only continue to increase in the future). In the absence of negative forcings, CO2 would have contributed 0.79°C of the 0.8°C observed global surface temperature rise, and hence we would expect the total observed rise to be double that. This tells us that the negative forcings (primarily from human aerosol emissions) have offset approximately 50% of the net positive forcings.

We also found that the net anthropogenic radiative forcing (sum of all positive and negative forcings) accounts for approximately 80% of the observed average surface warming over the past century (~0.65 out of 0.8°C). The other ~20% is a combination of natural forcings (primarily solar), and perhaps a bit of natural variability.

Another key point is that aerosols have a short atmospheric lifetime, unlike long-lived greenhouse gases. Thus their large offsetting of close to 50% of the net positive radiative forcing is only temporary, and will decline rapidly if we reduce aerosol emissions. This is why, as Isaac Held noted in the quote above, we fully expect CO2 and other greenhouse gases to continue as the dominant cause of global warming, and why although we need to address other issues like land use change, CO2 emissions are rightfully the primary target in mitigating climate change.

I think there's a weird kind of thinking that can flow from the fractioning of various influences to global warming. Since CO2 is the main culprit, it has become a kind of stand-in for all human contributions. So, if you diminish the role of CO2 you can pretend that it really isn't that big a deal. But soot is also significant contributor and all too human in its provenance. Fix one (clear coal out of our energy production) and you can go a long way to fixing both. But our carbon-extraction overlords have pointedly gone out of their way to make sure we don't address either. Any thing and everything can be used to make delay seem a viable alternative.

1. NRC (2005)- see http://www.nap.edu/openbook.php?record_id=11175&page=40 concluded that several aerosol effects were positive. I chose the nominal value of +0.1 Watts per meter squared. I agree these are uncertain, but conclude we do not know more about them in 2011 than we did then. A review paper on this specific topic would be quite informative.

2. I can accept your conclusion of double-counting and use the ~32% value. The 4% difference is certainly within the envelope of uncertainty in any case.

If you then start from the 52.4% value that you list from IPCC AR4, our improved understanding of the role of black carbon (in the atmosphere and at the surface) and the other aerosol effects, this will reduce this value. We seem to be in agreement that the black carbon positive radiative heating is larger than assumed previously. This has already reduced the contribution to 48.2% in your value for Skeie (2011).

If we use the 0.9 Watts per meter squared value for the black carbon that you present,from the Ramanathan and Carmichael (2008) values and the 0.2 Watts per meter squared as reasonable estimates based on the NRC (2005) report, which is still an accurate summary of our limited knowledge of this forcing, I calculate that CO2 is ~40% for the anthropogenic positive radiative forcing. This accepts the values for ozone that are listed in your table.

The CO2 forcing, to my knowledge, does not consider humid regions, such as the tropics, where existing water vapor reduces the effective radiative forcing due to the CO2 by itself. If this is not included, it would reduce the radiative forcing fraction from CO2 further. Please clarify.

3. You also did not discuss that CO2 has been increasing for over a century and some of the CO2 radiative forcing during that period would have been accomodated by the warmer climate. As I wrote, in 2005 V. Ramanthan replied to me (in an estimate) that about 20% of the difference between pre-industrial and current radiative forcing would have been accomodated. Thus the current radiative forcing from added CO2 would need to be reduced by this amount. The other positive radiative forcings, being more recent presumably are not reduced as much. As an estimate, we use a 20% reduction for the CO2 forcing, this lowers the fraction of positive radiative forcing from CO2 to ~35% of the anthropogenic radiative forcing.

This is still a back of the envelope calculation and I have proposed the following in my weblog post

1. Use a column radiative transfer model (for all wavelengths - i.e. short and long wave) on a vertical profile of temperature, humidity and clouds at a sufficient number of locations (grid points) around the world (using all global reanalysis grid points) during a year (with hourly time intervals) to determine the baseline current radiative forcing. If resouces permit, do more than one year. Calculate the global average radiative forcing by integrating over the year at each grid point. While radiative feedbacks, of course, are implicit in the vertical profiles, the radiative transfer model provides the instantaneous forcing at that time.

2. Use the column radiative transfer model with these same soundings but [sensitivity test #1] change the CO2 level back to pre-industrial, [sensitivity test #2] change the aerosol load back to preindustrial, [sensitivity test #3] change the land cover back to natural, etc and express the values in Watts per meter squared.

3. For each of these sensitivity tests, sum up the differences in radiative forcings to obtain the global annual average in Watts per meter squared.

The important conclusion, in my view, is that added CO2 is a substantial positive radiative forcing but that it is not currently (or in the past) been the majority of positive radiative forcing.

We agree that it will be increasing its fractional contribution in the future and certainly could become the majority if the aerosol input were eliminated.

4. However, in your take-home message, you have not answered my question in my weblog post with respect to the following:

"My question back to Skeptical Science, is to present your perspective on this issue. How would you propose including the assessment of the effect of each of the human climate forcings in terms of their effects on regional atmospheric and ocean circulations (i.e. by altering the pressure gradients through diabatic heating due to the radiative heating/cooling from CO2 and the other greenhouse gases, aerosols and land use/land cover change?

I recommend as a starting point for discussion on this issue, our paper

"This paper diagnoses the spatial mean and the spatial gradient of the aerosol radiative forcing in comparison with those of well-mixed green-house gases (GHG). Unlike GHG, aerosols have much greater spatial heterogeneity in their radiative forcing. The heterogeneous diabatic heating can modulate the gradient in horizontal pressure field and atmospheric circulations, thus altering the regional climate. For this, we diagnose the Normalized Gradient of Radiative Forcing (NGoRF), as a fraction of the present global heterogeneous insolation attributed to human activity. Although the GHG has a larger forcing (+1.7 Wm-2) as measured than those of aerosol direct (-1.59 Wm-2) and possible indirect effect (-1.38 Wm-2) in terms of a spatially averaged top-of-atmosphere value, the aerosol direct and indirect effects have far greater NGoRF values (~0.18) than that of GHG (~0.003)."

What is the SkS response to this perspective? In terms of how weather patterns are affected, in my view, heterogenous radiative forcings are much more important than the global average radiative forcing. I would like to see you discuss this issue.

5. In your take-home message, you also still have not answered the question as to what is the current (2011) positive radiative forcing? Also, what is total current radiative forcing and the current radiative feedback?

6. In answer to your question

"Approximately what percentage of the global warming (increase in surface, atmosphere, ocean temperatures, etc.) over the past 100 years would you estimate is due to human greenhouse gas emissions and other anthropogenic effects?"

where we show how important is land use/land cover change. If one accepts the model results [although this, of course, does not include aerosol effect and natural forcings and feedbacks), about one half of the trends over land can be explained by land use change - where the effect is to reduce the warming effect from the CO2. Other studies, e.g. see

suggests that land use change dominates surface temperature in that region but in this case there is a positive temperature effect from the land conversion.

My bottom line summary:

1. We both agree that the radiative forcing of added CO2 is an important climate forcing. While we disagree on its fractional contribution at present, we both agree this fraction will become larger in the future (as will its biogeochemical forcing).

It is also a long lived change to the composition of the Earth's atmosphere, and thus its reduction (removal) is a duanting challenge. My son, in his book The Carbon Fix, discusses carbon capture directly from the atmosphere as one possible option.

However, other climate forcings are also long-lived. This includes land use/land over change, and nitrogen deposition. Both will influence atmospheric processes. For a sobering view of how much we are altering the climate system from nitrogen deposition, see slide 12 in http://pielkeclimatesci.files.wordpress.com/2011/09/ppt-121.pdf

2. The positive radiative forcing from soot (black carbon) in the atmosphere and at the surface is larger than assumed in the past. The radiative forcing from other aerosols remains uncertain but several of the effects are considered by some to be positive and significant.

3. From a policy perpsective, our debate is essentially irrelevant. We both agree that permitting atmospheric concentrations to increase risks consequences.

Where we disagree is i) I have concluded it is the biogeochemical forcing of added CO2 that is the larger concern with respect to the CO2, and ii) human-added CO2 is a first order climate forcing but is not the only first order human climate forcing. In terms of what matters most to society and the environment, the more heterogenous climate forcings (both in terms of regional radiative forcing and non-radiative effects) should be elevated in concern.

This later view is of policy relevance as Mike Hulme has written on (see slide 71 in
http://pielkeclimatesci.files.wordpress.com/2011/09/ppt-121.pdf)

Dr. Pielke - "You also did not discuss that CO2 has been increasing for over a century and some of the CO2 radiative forcing during that period would have been accomodated by the warmer climate. As I wrote, in 2005 V. Ramanthan replied to me (in an estimate) that about 20% of the difference between pre-industrial and current radiative forcing would have been accomodated. Thus the current radiative forcing from added CO2 would need to be reduced by this amount."

I would have to strongly disagree - the radiative forcings discussed here (as in the IPCC AR4, the basis of this discussion) are relative to 1750 pre-industrial forcings, not the current imbalance (unrealized warming) between temperatures and those forcings.

The 20% adjustment you recommend here is an odd (and IMO quite unwarranted) redefinition of well understood terms - shifting the baseline. Unrealized warming and current imbalance are different terms, different values, than forcing changes since 1750.

If you are looking at unrealized warming and remaining imbalances (rather than the changes since 1750 that are the topic of this thread), it's noteworthy that you cannot just scale the change in CO2 contribution - all radiative imbalances scaled by unrealized warming would be scaled as well, meaning that the relative contribution of CO2 should not change. Your 20% reduction of relative CO2 contribution is, again, not justified in my view.

KR- My question remains, what is the current radiative forcing of CO2? In the SPM for the IPCC 2007 report, with respect to their presentation of the values in their figure SPM.2 they write

"Global average radiative forcing (RF) estimates and ranges in 2005 for anthropogenic carbon dioxide (CO2 ), methane (CH4 ), nitrous oxide (N2O) and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientifi c understanding (LOSU)."

They specifically state "Global average radiative forcing (RF) estimates and ranges in 2005".

Then in footnote #2, they write

"In this report, radiative forcing values are for 2005 relative to pre-industrial conditions defined at 1750 and are expressed in watts per square metre (W m–2). "

At the best, this was sloppy writing (as it is corrected/clarified in the footnote), but the figure caption itself is misunderstood by quite a few people. At worst, the writers were not clear oo this when they wrote the figure caption.

KR - I agree this scaling would be the same if all of the forcings had the same time evolution. When I discussed this with V. Ramanathan and others on our committee, however, the conclusion was that the other forcings ramped up more recently.

In any case, what we really need is the current best extimate of the 2011 global-annual averaged radiative forcing and the best estimate of the 2011 global-annual averaged radiative imbalance. The difference between these two values is the global-annual averaged radiative feedback.

Dr. Pielke, the question we are addressing with this post is the CO2 (and net anthropogenic) contribution to the observed surface warming over the past century. In order to answer this question, we must examine the change in forcings over that period of time (not the remaining imbalance/unrealized warming, which is a separate issue).

You have not identified any problems with our calculations of the CO2 contribution over the past century - namely accounting for your calculational errors and correcting values for the methane, ozone, and albedo forcings brings the CO2 contribution to the net positive radiative forcing over this period to ~50%. We also showed that CO2 caused ~0.79°C, and the net anthropogenic forcing caused ~0.65°C surface warming over the past century. We again ask if you now agree with these values (and the ranges for these values listed in the post above).

The issues you raise regarding the CO2 contribution to the more recent radiative imbalance is a separate question, which does not affect the calculation of the total CO2 contribution over the past century. We can proceed to discuss this issue as well, but first would like to close the discussion on the human-caused warming over the past century.

dana1981 - We know too little about the role of natural variations in the radiative forcing in the last century to know with such precision how much of that warming is from anthropogenic effects. This is based, in part, on the research and comments of Roy Spencer, Judy Curry, Judith Lean and others. [this is one reason I recommend you focus on science and not Roy's policy/political statements; he has added important new insight into the role of natural forcings including solar and internal multi-year variability].

Even with the anthropogenic forcings, we do not know what the actual aerosol and land use/land cover changes have contributed, relative to the radiative effect of added CO2, with respect to the observed surface temperature trends. There is also the issue of siting quality for the land portion of the surface temperature data.

I agree that human's have significantly affected the annual average surface temperature trends, but, in my view, the issue as you present above inadequately considers all of the issues.

Thus, I suggest we move on. I do not find this an important issue, but would be open to you explaining why it is. It seems to me that knowing the current forcings is much more relevant.

P.S. I would like you to tell us if the water vapor/CO2 overlap was considered in your calculation. I do not see any problem in your calculation, if your numbers are used. We disagree with the values, however, as I wrote in my response.

"If we use the 0.9 Watts per meter squared value for the black carbon that you present,from the Ramanathan and Carmichael (2008) values and the 0.2 Watts per meter squared as reasonable estimates based on the NRC (2005) report, which is still an accurate summary of our limited knowledge of this forcing, I calculate that CO2 is ~40% for the anthropogenic positive radiative forcing. This accepts the values for ozone that are listed in your table."

Dr. Pielke, if you had read SkS' posts, you would realize we have examined Dr. Spencer's scientific research quite extensively.

However, this calculation is essentially based on two factors - transient climate sensitivity, and the CO2/net anthropogenic forcings. Internal variability does not factor into the calculation of how much warming these forcings have caused.

I agree, the aerosol forcing in particular represents a significant uncertainty. That's why we have been very explicit that we're stictly looking at the best estimates of these forcings.

However, the CO2 forcing is very well-known, and we provided a 90% confidence range on the transient climate sensitivity parameter. Surely you can thus at least agree that CO2 has caused a 0.64 to 1.28°C (with a best estimate of 0.79°C) warming of average global surface temperature over the past century?

Third, from Ramanathan and Carmichael (2008):"The TOA BC forcing implies that BC has a surface warming effect of about 0.5 to 1 °C, where we have assumed a climate sensitivity of 2 to 4 ºC for a doubling of CO2. Because BC forcing results in a vertical redistribution of the solar forcing, a simple scaling of the forcing with the CO2 doubling climate sensitivity parameter may not be appropriate".

So they concede that their method may not be appropriate, or does Dr. Pielke wish us to forget/neglect this important caveat from their paper which he chose to cite?

Now perhaps you are asserting that statements such as "warming in recent decades is mostly due to a natural cycle in the climate system — not to an increase in atmospheric carbon dioxide from fossil fuel burning" are political in nature, in that they don't follow from any robust scientific analysis.

SkS has done a pretty comprehensive overview of Dr Spencer's contribution. However, he also authored numerous other pieces, opinions or statements that are commonly used to justify skepticism. It is entirely wihin the scope of SkS' stated vocation to examine these other productions of his and see how they compare with the existing science, including his own. As far as I have read, that is what has been done on SkS.

I would like to remind readers that the purpose of SkS is not to assess the state of the science and identify areas of greater or lower uncertainty, or recommend avenues for further research. Not that such questions should not be given attention here. Ideally, it would be possible to indeed focus only on true, interesting scientific problems. However, SkS was created in response to the tremendous effort of disinformation and propaganda that this particular area of science has unfortunately experienced.

The purpose of SkS is to examine common claims put forth by self proclaimed skeptics who doubt all or part of well accepted conclusions reached by mainstream climate science. These claims are examined and weighed in regard to what the published science reveals to date on the subject. In that sense, all skeptic claims are fair game to SkS, whether they can be labeled as "policy/political statements" and regardless of their source.

Albatross - The values I present were given to show reasonable deductions from the IPCC value and that of Skeie 2011. I do not know the precise value (and neither does anyone) but the fraction is clearly well less than 50% using reasonable values.

In terms of the statement

"a simple scaling of the forcing with the CO2 doubling climate sensitivity parameter may not be appropriate".

this applies to the entire question on the fraction of positive radiative forcing between 1750 and currently.

Tom Curtis - The 0.2 Watts per meter squared is for the two indirect aerosol effects that are reported on in the 2005 NRC report. This is an estimate based on them being positive. There is clearly an uncertainty on their value but the report concluded they were positive. New research remains unclear on their magnitude.

NewYorkJ - In the comments on my earlier posts on SkS, there were quite a few comments on his politics. Where has SkS positively recognized his finding on a larger natural influence, even if you (and I) disagree with Roy that the warming was mostly natural? His basic idea is sound and a significant scientific advancement.

While disagreeing with Albatross's apparent imputation that more recent papers automatically trump earlier papers, never-the-less it seems intemperate to dismiss the Skeie et al values as unreasonable. Given that they are reasonable values, and that the contribution of CO2 to the anthropogenic forcing using Skeie et al values is 48.2%, I do not believe your claim that "the fraction is clearly well less than 50% using reasonable values" is justified.

If you disagree that the Skeie et al values are reasonable, perhaps you would have the courtesy to explain why rather than simply dismissing them from the range of "reasonable values".

Tom Curtis - I presented a summary of why I have concluded ~50% is too high. I also have emphasized that it is not an important issue. Part of the difference is that, as reported in http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-3.html#2-3-1

the contribution due to CO2 has increased 20% just between 1995 to 2005; i.e.

"....In the decade 1995 to 2005, the RF due to CO2 increased by about 0.28 W m–2 (20%), an increase greater than that calculated for any decade since at least 1800...."

However, in looking through this chapter, I do not see where they considered the water vapor overlap. This would lower the fraction.

They also write

"Using the global average value of 379 ppm for atmospheric CO2 in 2005 gives an RF of 1.66 ± 0.17 W m–2; a contribution that dominates that of all other forcing agents considered in this chapter."

so they do explicitly state that this forcing was for 2005 not the difference from pre-industrial.

Also, where is the water vapor/CO2 overlap considered? This would be in the models, but I do not see this evaluation in the IPCC chapter and in the SPM figure. I am hoping someone at SkS can clarify.

At a more fundamental level, what difference does it make if it is 25% or 50%? My interpretation of the published papers came up with a smaller fraction. SkS and the IPCC have a larger fraction. Would you propose different policy if it were a lower fraction? It will be increasing in the future in any case. The fraction make no difference in the modeling since it is part of their calculations.

Each of the approaches presented so far on SkS and in the IPCC, as well as my back-of-the-envelope list, to estimate these values are inadequate. I have proposed a way to better assess these numbers, but have had no feedback on that so far from SkS.

I have also asked a number of other questions and will have more on my weblog tomorrow.

>>Where has SkS positively recognized his finding on a larger natural influence, even if you (and I) disagree with Roy that the warming was mostly natural? His basic idea is sound and a significant scientific advancement.

It seems you are presenting a moving target argument here. In post 11, you write "this is one reason I recommend you focus on science and not Roy's policy/political statements." When it is pointed out that SkS did review Spencer's work, you move the target to claim that SkS must do so in a positive way.

More importantly, Spencer's basic idea is hardly sound and significant, which is why SkS has not reviewed it "positively." However, Roy Spencer is hardly the topic of this thread. If you think that Spencer in any way undermines the post by SkS, could you please cite his specific works in specific ways? That would be helpful to clarifying the "the contribution of CO2 to the net positive anthropogenic radiative forcing," the topic of this discussion.

It strikes me that you again want to change the conversation when others dispute your claims. The difference between 20% and 50% is quite large, and you in fact brought up this figure to prove your point. You end your posting by saying you will ask more questions, but what are the use of you raising questions, when you simply won't stand for any in depth debate in trying to answer these questions, but want to change the subject each time?

"The simple formulae for RF of the LLGHG quoted in Ramaswamy et al. (2001) are still valid. These formulae are based on global RF calculations where clouds, stratospheric adjustment and solar absorption are included, and give an RF of +3.7 W m–2 for a doubling in the CO2 mixing ratio. (The formula used for the CO2 RF calculation in this chapter is the IPCC (1990) expression as revised in the TAR."

(My emphasis)

Referring back to the IPCC TAR, we find the adjustment to the simple formula was due to the work of Myhre et al, 1998, which in turn depends on intermodel comparisons performed in Myhre and Stordal, 1997 (hereafter, M&S97).

In M&S97, Myhre and Stordal perform a detailed series of comparisons between LBL models and a Broad Band Model used at various resolutions. The most detailed resolution used a 2.50x2.50 grid. The coarsest used a global mean climatology. M&S97 explicitly state that:

"Overlap is considered between gases that absorb in the same spectral region."

They go on to describe how the overlap is handled. As the 2.50x2.50 is global in extent, it necessarily includes the highly humid tropics, and more significantly the very cool polar regions. Later M&S97 compare the 2.50x2.50 model to a 100x100, a 2.50 zonal mean model, a 100 zonal mean model, and a global mean climatology model. The difference in forcing between each of these models is less than 1% in all cases.

Further on they explicitly compare CO2 radiative forcing with altitude for Tropical, Mid-Latitude Summer, and Sub-Arctic Winter conditions using both the LBL and Broadband model. The Tropical and Mid-Latitude Summer forcings are scarely distinguishable, with the MLS forcing being slightly stronger. The SAW forcing is considerably weaker than either the TROP or MLS forcing. Of course, as the formula is based on a full global comparison, that is of no consequence to the final figure.

2) With regard to whether the IPCC AR4 quotes transient forcings in a given year, or the forcing relative to preindustrial levels, I refer you to this chart:

Note the charts heading. For greater clarity, the caption reads:

"FAQ 2.1, Figure 2. Summary of the principal components of the radiative forcing of climate change. All these radiative forcings result from one or more factors that affect climate and are associated with human activities or natural processes as discussed in the text. The values represent the forcings in 2005 relative to the start of the industrial era (about 1750). Human activities cause significant changes in long-lived gases, ozone, water vapour, surface albedo, aerosols and contrails. The only increase in natural forcing of any significance between 1750 and 2005 occurred in solar irradiance. Positive forcings lead to warming of climate and negative forcings lead to a cooling. The thin black line attached to each coloured bar represents the range of uncertainty for the respective value. (Figure adapted from Figure 2.20 of this report.)﻿"

(My emphasis)

Lest there be any doubt, based on a pixel count the chart shows a CO2 radiative forcing of 1.67 W/m^2, in agreement with the text.

As to what difference this makes, not a great deal. Never-the-less, you brought the question up. You have blogged on the issue at least twice, and have claimed repeatedly that the CO2 radiative forcing is over-estimated when you have in fact been under estimating it. And you have presented your significant underestimate based on your back of an envelope calculation in a talk to a scientific conference. I would have thought that, given the circumstances, professional pride alone would make you wish to correct those errors with alacrity.

My old professor used to tell me that if I couldn't present what I was trying to say on a single side of A4, then I didn't really understand what I was trying to say. Given the length of Pielke Sn's replies to this post, I here attempt to summarise them. If the summary is poor it is (with due respect) because the replies are so poorly structured and verbose.
This SkS post is asking Pielke Sn two questions
(1) Is his comment that CO2 is but 26% or 28% of human-caused forcing wrong & actually more like 50%?
Pieke Sn @ 4 replies that given the numbers, the figure should be 40% but is not sure if this includes all humidity effects. Pieke Sn then guestimates the figure would be 35% if the CO2 forcing was taken as the present-day radiative imbalance. Pielke Sn sees it as important that we understand that the contribution of CO2 to the warming is less than 50% (for unstated reasons) but also sees the proportion of CO2's contribution rising higher in future. He asks SkS what it thinks of NGoRF. He also points to the present-day radiative imbalance & associated feedback figures not yet given by SkS. Pielke Sn asks for these figures (in @7 & @11) asserting the present-day radiative imbalance is the important factor. And so CO2 induced warming in past years he sees as unimportant without a good reason being given. Pielke Sn considers the % of human-caused forcing due to CO2, be it 25% or 50%, irrelevant (@22)
(2) What temperature rise over the last 100 years is down to human activity?
Pielke Sn points to a reference where land use is an important factor but gives no direct answer, and (@11) that too little is known to say, although it was “significant”.

Piekle Sn concludes CO2 is important to climate forcing and mentions its removal from the atmosphere. He mentions other causes of human-caused forcing, emphasising that these should have elevated concern as this matters most to society and the environment (for unstated reasons).

MA Rodger - Regarding your question #1, both estimates are likely wrong. As I have written, there are several unresolved issues on how to calculate this fraction.

On question #2, the change in surface temperatures over the last 100 years is a still poorly understood mix of added CO2, aerosols, land use/land cover change, poor siting of land data, solar influences, volcanoes and internal long term climate variability.

Tom Curtis - Your extract from the IPCC AR4 adds some information on the water vapor overlap issue. However, it still does not quantify how much of the CO2 radiative forcing is not occuring due to the water vpaor overlap.

The statement

""The simple formulae for RF of the LLGHG quoted in Ramaswamy et al. (2001) are still valid. These formulae are based on global RF calculations where clouds, stratospheric adjustment and solar absorption are included, and give an RF of +3.7 W m–2 for a doubling in the CO2 mixing ratio. (The formula used for the CO2 RF calculation in this chapter is the IPCC (1990) expression as revised in the TAR."

is incomplete as I do not see "water vapor" listed.

Here is the simple question: What would be the global annual average radiative forcing change since pre-industrial with CO2 without the water vapor overlap and with the the overlap?

On whether the figure is interpreted as the 2005 radiative forcing or the difference since preindustrial, I agree it is the later. The figure caption and text I quote said otherwise. The FAQ you listed was a correction to the original SPM [which still contains the erroenous information].

paulhtremblay - I never said it was 20%. My estimate was higher than that but significantly lower than 50%. I presented a way to resolve this issue.

It seems, however, that the comments on this thread has deteroriated again, as instead of answering my questions, you (and others) keep insisting that I agree with your view, even when I present information/questions that conflict with your statements.

For example, why does it matter if the fraction of radiative forcing in 2005 compared with pre-industrial was 28% or 48%? My analysis suggests a smaller fraction but it is increasing with time. However, why do we care? Its biogeochmeical effect is directly connected to its atmospheric concentration and we know that much better than we know the global average radiative forcing.

By focusing on such trivial questions as this fraction, the really important science questions which I have raised are being ignored on SkS.

Professor Pielke used a value of 1.1 W/m2 not 1.4 W/m2 for the CO2 forcing.

He did this to leave the total forcing from greenhouse gases unchanged. After increasing the forcing for CH4 he was obliged to decrease the forcing for CO2.

"By summing the 0.8 Watts per meter squared for methane and using the total of 2.4 Watts per meter squared of the well-mixed greenhouse gases from the IPCC Report, the radiative contribution of CO2 reduces to about 46% of this component of radiative forcing (1.1 Watts per meter squared)."

As was pointed out on real climate at the time this is totally unjustified since the forcing effect of CO2 is independently derived.

I agree with MA Roger #26 that Dr Pielke's responses are somewhat confusing.

In all these discussions of Radiative forcing using the AR4 chart, the 'Human Activity' forcings are referenced to year 1750, when it is assumed that these forcings were 'insignificant' ie: zero.

That means that all these forcings are absolute numbers baselined to zero. This might answer Dr Pielke's confusion.

As Tom Curtis points out from AR4 "The only increase in natural forcing of any significance between 1750 and 2005 occurred in solar irradiance."

The problem with including solar forcing in the sum of the AR4 chart has been pointed out by others in SKS threads in that we don't know if there was a positive or negative planetary warming imbalance in 1750, and whatever it was - it could only have come from solar irradiance since all the 'human activity forcings' are zero.

It is likely that as the Earth warmed out of the little ice age, the solar irradiance imbalance was positive - not zero, so the AR4 value of 0.12W/M2 should be added to whatever the 1750 value was in order to get a comparable absolute value in 2005.

Further, the climate responses are not included in the AR4 chart and Dr Trenberth has calculated these at a net minus (-)0.7W/M2 which brings the net warming imbalance down to +0.9W/M2.

It should be noted that Dr Trenberth uses a figure of minus (-)2.8W/M2 for radiative cooling (stefan-boltzman) and +2.1W/M2 for water vapour and ice albedo feedback to arrive at the net minus (-)0.7W/M2 climate response.

Of course the +0.9W/M2 is also in dispute in recent times due to Dr Hansen's claimed increased aerosol reflectivity and effective reduction of the warming imbalance to about +0.6W/M2.

The point to be made here is that since 1750, all the increasing 'human activity' forcings and climate responses have acted together producing a continuously changing net imbalance forcing, the sum total of which integrated over time will represent the net energy gained by the planet. Most of this energy must be sequestered in the oceans and represented by past temperature increase and phase changes in ice or water.

Arguing the proportions of CO2 forcing to the percentage point without accurately knowing the historical forcings from solar, aerosols and the feedbacks is somewhat academic.

critical mass - We do not need to know the historical forcings to estimate the current (2011) radiative forcing from all sources and the current radiative imbalance (using ocean heat storage changes). This is one of the first questions that should be answered in a climate change assessment.

1) I recommend that you reread my post @25, or better yet, Myhre and Stordal, 1997. As clearly indicated in my post, it was Myhre et al, 1998 who determined the strength of the CO2 radiative forcing by determining the value of the constant in the simple formula for radiative forcing. In doing so they corrected downwards the factor previously used from 6.3 to 5.35.

As explained previously, Myrhe et al is built on the detailed model comparisons in Myrhe and Stordal 97, which include a global model run at a 2.5o x 2.5o resolution. That model, because global necessarily included the difference in radiative transfer between tropical and not tropical regions. To further clarify the point, I noted that M&S97 had also run both the broadband model and the LBL model for both tropical and mid-latitude summer conditions, with the latter showing the stronger forcing, clearly showing the effect of increased humidity and cloud cover had been included.

"IPCC (1990) and the SAR used a radiative forcing of 4.37 Wm-2 for a doubling of CO2 calculated with a simplified expression. Since then several studies, including some using GCMs (Mitchell and Johns, 1997; Ramaswamy and Chen, 1997b; Hansen et al., 1998), have calculated a lower radiative forcing due to CO2 (Pinnock et al., 1995; Roehl et al., 1995; Myhre and Stordal, 1997; Myhre et al., 1998b; Jain et al., 2000). The newer estimates of radiative forcing due to a doubling of CO2 are between 3.5 and 4.1 Wm-2 with the relevant species and various overlaps between greenhouse gases included. The lower forcing in the cited newer studies is due to an accounting of the stratospheric temperature adjustment which was not properly taken into account in the simplified expression used in IPCC (1990) and the SAR (Myhre et al., 1998b). In Myhre et al. (1998b) and Jain et al. (2000), the short-wave forcing due to CO2 is also included, an effect not taken into account in the SAR. The short-wave effect results in a negative forcing contribution for the surface-troposphere system owing to the extra absorption due to CO2 in the stratosphere; however, this effect is relatively small compared to the total radiative forcing (< 5%)."

(My emphasis)

The Fourth Assessment Report contented itself with saying:

"The simple formulae for RF of the LLGHG quoted in Ramaswamy et al. (2001) are still valid. These formulae are based on global RF calculations where clouds, stratospheric adjustment and solar absorption are included, and give an RF of +3.7 W m–2 for a doubling in the CO2 mixing ratio. (The formula used for the CO2 RF calculation in this chapter is the IPCC (1990) expression as revised in the TAR. Note that for CO2, RF increases logarithmically with mixing ratio.) Collins et al. (2006) performed a comparison of five detailed line-by-line models and 20 GCM radiation schemes. The spread of line-by-line model results were consistent with the ±10% uncertainty estimate for the LLGHG RFs adopted in Ramaswamy et al. (2001) and a similar ±10% for the 90% confidence interval is adopted here. However, it is also important to note that these relatively small uncertainties are not always achievable when incorporating the LLGHG forcings into GCMs. For example, both Collins et al. (2006) and Forster and Taylor (2006) found that GCM radiation schemes could have inaccuracies of around 20% in their total LLGHG RF (see also Sections 2.3.2 and 10.2)."

(My emphasis)

Ramaswamy et al, 2001 is of course, the IPCC TAR. The IPCC do not feel it necessary to spell out details that have been public knowledge for six years (as at the time of the AR4), contenting themselves with a reference to the original discussion.

Now, given the detailed analysis by Myhre and Stordal and the explicit statement by the TAR, do you still wish to maintain that the radiative forcing as calculated does not allow for overlap with H2O in the tropics?

2) The passage you quoted, it was not a figure caption, did not say otherwise, it just did not specify a fact that was well known. Further, the FAQ plus Fig 2 of the FAQ which I reproduced was not added afterwards. It can be found on page 135 of the PDF reproduction of the original report for anybody interested. What is more, the figure 2.20A on which it is based (page 203) has the same heading.

"The combined anthropogenic RF is estimated to be +1.6 [–1.0, +0.8][2] W m–2, indicating that, since 1750, it is extremely likely[3] that humans have exerted a substantial warming influence on climate. This RF estimate is likely to be at least five times greater than that due to solar irradiance changes. For the period 1950 to 2005, it is exceptionally unlikely that the combined natural RF (solar irradiance plus volcanic aerosol) has had a warming influence comparable to that of the combined anthropogenic RF."

"Changes in the atmospheric abundance of greenhouse gases and aerosols, in solar radiation and in land surface properties alter the energy balance of the climate system. These changes are expressed in terms of radiative forcing,[2] which is used to compare how a range of human and natural factors drive warming or cooling influences on global climate. Since the TAR, new observations and related modelling of greenhouse gases, solar activity, land surface properties and some aspects of aerosols have led to improvements in the quantitative estimates of radiative forcing."

You will notice the footnote after the introduction of the term "Radiative Forcing". That footnote reads:

"Radiative forcing is a measure of the influence that a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. Positive forcing tends to warm the surface while negative forcing tends to cool it. In this report, radiative forcing values are for 2005 relative to pre-industrial conditions defined at 1750 and are expressed in watts per square metre (W m–2). See Glossary and Section 2.2 for further details."

Should we check the glossary as well, or is my point sufficiently made?

This is a very minor, an absolutely trivial point, except for one factor. It is one thing for a Professor of Climatology with, I must add, a very distinguished career, to make a simple mistake on a fact you would expect him to know well. It is quite another to try and save face by making "facts" up. Knowledge is not so often found in this world that it can be thrown away in face saving excercises.

Tom Curtis - You seem to persist in missing my issue. I know the models have been applied which include the water vpaor and CO2 overlap. However, I have not seen this reported using the 1-D radiative transfer calculations as I proposed. I repeat my question (and stop referring to papers - I am asking a straightforward question):

What would be the global annual average radiative forcing change since pre-industrial with CO2 without the water vapor overlap and with the the overlap?

On the IPCC values, I agree that they are presenting the difference between pre-industrial and 2005. That is not in dispute. They are inconsistent, however, in terms of how they write this in the text in places as they specifically write, for example, in SPM.2

"Global average radiative forcing (RF) estimates and ranges in 2005"

[http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf]

This is a trivial issue, except that i) the statement is incorrect as it is not the "forcing" in 2005 and ii) quite a few people accept that the values in the figure are the current forcings.

Finally, if I am going to continue this discussion with you, keep your snarky comments out of your posts. That is why I left SkS before. We disagree. That's the way science goes. But when you start with insults because I do not accept your view, I will go off to where more constructive debating occurs.

But when you start with insults because I do not accept your view, I will go off to where more constructive debating occurs.

As a matter of interest (and certainly not in a snarky or insulting manner), can you give any examples of where you would get such debating - if you have online sources in mind ? I am genuinely interested.

"The values I present were given to show reasonable deductions from the IPCC value and that of Skeie 2011. I do not know the precise value (and neither does anyone) but the fraction is clearly well less than 50% using reasonable values."

Let's examine where we're at in this discussion. You originally argued that CO2 was only responsible for 26-28% of the net positive radiative forcing. We found some mathematical errors in your calculations which bring the value up to ~30%, and you concur with these corrections. We also identified a 0.3 W/m2 error in your methane estimate, a 0.5 W/m2 error in your albedo estimate, a 0.3 W/m2 error in your ozone estimate, and a 0.4 W/m2 error in your CO2 estimate - you appear to concur with all of these corrections, with some caveats on CO2. These corrections bring the value up around our original estimate of 50%.

You then claimed that 20% of the CO2 forcing has been "accomodated by a warmer climate" based on a personal communication. We have several issues with this claim, but regardless, it is not relevant to the question at hand (the CO2 contribution over the past century).

Your only other revisions to the Skeie estimate are to use the Ramanathan and Carmichael best estimate for the black carbon forcing of 0.9 W/m2 (without justification for this choice, which also conflicts with your previous claim that the black carbon forcing is 0.5 W/m2), an assertion of a 0.2 W/m2 aerosol forcing, and an assertion that the water/CO2 overlap in the tropics has been ignored. As Tom Curtis has noted, the overlap was addressed in seminal work by Myhre, and incorporated into the IPCC reports.

There is reason to believe the Ramanathan BC forcing is too high (i.e. see Bond 2011 and another paper by Skeie et al. 2011), but incorporating their BC value and your aerosol forcing estimates, the CO2 contribution is still between 41% and 50%, and certainly far greater than your originally asserted 26%. So we can emphasize the point that any reasonable calculation based on the agreed-upon inputs will put the CO2 contribution to the net positive forcing at 41% to 50% and rising.

You now claim that whether the value is 25% or 50% does not matter, yet you have frequently raised the issue on your blog and in presentations and talks.

Ultimately we have identified a number of errors in your calculations, and yet you continue to insist that despite these corrections, somehow your argument must be correct. We have supported our estimate with detailed calculations and references, and have demonstrated that CO2 has thus far accounted for approximately 0.8C surface warming - a calculation which you have not disputed.

We are a bit disappointed that this refutation of your reasoning leaves your sense of conviction so unmoved, but at this point, we may as well move on to other issues. Readers can examine our calculations for themselves and decide who is correct.

1) A LBL (Line By Line) model as used by Myhre et al, 98, and Myhre and Stordal, 97 is a one dimensional radiative transfer model, and hence the papers to which I have been referring answer the general point you have been making. They, however, address the practical question of what the radiative forcing is in the real world which includes water vapour. They do not adress the hypothetical question that you ask, ie, what would the radiative forcing of CO2 be in the absence of water vapour. Because the question is purely hypothetical, it is irrelevant to future discussions so I have no interest in doing a literature search in the off chance that somebody has answered this hypothetical.

Please note that the radiative forcing of CO2 if there was no overlap with H2O, and the radiative forcing of CO2 in the absence of water vapour are the same, so the slightly different form in which I have expressed the question is of no consequence.

The global annual average radiative forcing of CO2 in the presence of H2O with overlaps accounted for in 2005 is the value given by the IPCC in AR4. The value in 2011 is that given by Skeie et al.

2) From the glossary of IPCC AR4:

"Radiative forcing Radiative forcing is the change in the net, downward minus upward, irradiance (expressed in W m–2) at the tropopause due to a change in an external driver of climate change, such as, for example, a change in the concentration of carbon dioxide or the output of the Sun. Radiative forcing is computed with all tropospheric properties held fixed at their unperturbed values, and after allowing for stratospheric temperatures, if perturbed, to readjust to radiative-dynamical equilibrium. Radiative forcing is called instantaneous if no change in stratospheric temperature is accounted for. For the purposes of this report, radiative forcing is further defined as the change relative to the year 1750 and, unless otherwise noted, refers to a global and annual average value. Radiative forcing is not to be confused with cloud radiative forcing, a similar terminology for describing an unrelated measure of the impact of clouds on the irradiance at the top of the atmosphere.

(My emphasis)

Therefore, according to the glossary, when the IPCC AR4 refers to the radiative forcing for 2005 they mean the change in radiative forcing in 2005 relative to 1750, unless they explicitly state otherwise. That could not be clearer.

What is more, the formula for radiative forcing of CO2 is given by the simple formula:

ΔF = αln(C/Co), where ΔF is the change in forcing, C is the CO2 concentration in the current year, Co is the CO2 concentration in the inital year, and α = 5.35 (source; also Myhre et al, 98, and various IPCC reports). This is the simple formula referred to in AR4. Clearly from its formula, the radiative forcing requires a baseline year. It is impossible to derive the radiative forcing from this formula for a single year simpliciter for the result would necessarily be 0. Consequently no interpretation of "Radiative Forcing" in AR4 in which it is treated as being the forcing in a single year is consistent with the text which explicitly refers to this simple formula.

In other words, not only are you in error in your interpretation of the IPCC AR4, logically your interpretation could not have failed to be in error. Note I say that you are in error because you insist on interpreting the IPCC AR4 as inconsistent whereas in fact you are simply failing to interpret their words in accordance with the glossary.

3) You object to what you call my snark. Well I object to the extreme lengths of misrepresentation you are prepared to go to cover up an error. Please note that "misrepresentation" is neither snark nor accusation. It simply notes that you have represented the facts to be one way (the IPCC AR4 was corrected; the IPCC AR4 is inconsistent) when transparently, and as could be discovered by simply reading a glossary, they were another way.

You and I are both here trying to reach SkS's audience in order to convince them of what we believe to be the truth about global warming. Fine, I am a great believer in the open market of ideas. But I will not accept a restraint on me that I must not correct your gross errors should they occur (and as has occurred) because such correction will offend our sensibilities. If you cannot debate under the condition that your mistakes will be corrected, then (speaking only for myself), I see little point in debating you.

JMurphy - I am able to present my viewpoint on my weblog. Colleagues (even those who disagree with me) reply via e-mail and I have posted a number of guest posts from such an interaction. As shown here, however, (and when I had comments), people often see they have an opportunity to become personal.

I never said I agreed with all your "corrections" (and you have even used my back-of-the-envelope estimate with solar included). You call them "errors" which hardly represents my view of the estimates. I just "accepted" them and then started with the higher fraction of positive radiative forcing from CO2 and presented other reasons it should be lower. From IPCC AR4 to Skeie 2011 it has been reduced from 52.4% to 48.2% [quite a bit of significance with three significant digits for such an imprecise quantity].

To also refer to the Myhre paper as the definitive statement ("seminal") is quite an overreach. The water vapor/CO2 overlap has not been completely addressed and I have repeated my question and will do so again;

What would be the global annual average radiative forcing change since pre-industrial with CO2 without the water vapor overlap and with the the overlap?

I also repeat

What is the current (2011) radiative forcing from each of the terms (including CO2) and what is the current radiative imbalance?

"A LBL (Line By Line) model as used by Myhre et al, 98, and Myhre and Stordal, 97 is a one dimensional radiative transfer model, and hence the papers to which I have been referring answer the general point you have been making"

I agree they addrees the "general point". It is my specific question that an answer is needed for

"What would be the global annual average radiative forcing change since pre-industrial with CO2 without the water vapor overlap and with the the overlap?"

I doubt you could find it in a literature search because to my knowledge, it has not been done. That is why we did an estimate in

Relative Roles of CO2 and Water Vapor in Radiative Forcing. http://pielkeclimatesci.wordpress.com/2006/05/05/co2h2o/

Further Analysis Of Radiative Forcing By Norm Woods
http://pielkeclimatesci.wordpress.com/2007/08/24/further-analysis-of-radiatve-forcing-by-norm-woods/

"Climate forcings can be classified as radiative (direct or indirect) or nonradiative. Direct radiative forcings affect the radiative budget of the Earth directly; for example, added CO2 absorbs and emits infrared (IR) radiation. Indirect radiative forcings create a radiative imbalance by first altering climate system components, which then almost immediately lead to changes in radiative fluxes; an example is the effect of aerosols on the precipitation efficiency of clouds."

This usage conflicts with your statements. In fact, you are inappropriate mixing a "forcing" from a "change in forcing over some time period". A forcing (such as produces an acceleration, is immediate).

3) Finally, I quess you and I are done. You are so utterly (dogmatically) convinced of your viewpoint, you are not interested in a contstructive debate. You claim to have shown I am wrong (my "gross errors) on a trivial issue, yet refuse to adequately answered a set of more substantive questions I have presented.

Very well Dr. Pielke. You may not agree with our corrections of your errors, but as I said, readers can decide for themselves who is right on this matter, since we have thoroughly documented the sources of our calculation and corrections in the post above.

"ΔF = αln(C/Co), where ΔF is the change in forcing, C is the CO2 concentration in the current year, Co is the CO2 concentration in the inital year, and α = 5.35 (source; also Myhre et al, 98, and various IPCC reports). This is the simple formula referred to in AR4. Clearly from its formula, the radiative forcing requires a baseline year. It is impossible to derive the radiative forcing from this formula for a single year simpliciter for the result would necessarily be 0."

If you would like us to answer your question, you will have to provide a baseline reference year.

I also agree with Tom's answer to your first question, also in comment #37:

"Because the question is purely hypothetical, it is irrelevant to future discussions so I have no interest in doing a literature search in the off chance that somebody has answered this hypothetical."

That does require a base year. The forcing does not and is instantaneous.

One would never state that "acceleration requires a base time period." Acceleration is the derivative of the velocity at any time. Similarly, radiative forcing is at a specific time although one could time average (e.g. the yearly global averaged radiative forcing).

>>The question is hardly "hypothetical" as the water vapor/CO2 overlap is a scientific issue.

So is the matter of radiative forcing. Yet, you write "For example, why does it matter if the fraction of radiative forcing in 2005 compared with pre-industrial was 28% or 48%?" That strikes me not only as anti-scientific, but hypocritical.

You imply that the Mhyre paper does not adequately address the overlap issue, but you are not at all specific, instead relying on a hypothetical question and to your own blog post (as opposed to a peer reviewed article) to provide some refutation, though even then, you remain vague as to how this overlap supports your original estimate of 28%.

"the radiative forcing requires a baseline year."
is not correct. The quote you have states
"ΔF is the change in forcing".
That does require a base year. The forcing does not and is instantaneous.

I will point out that, despite this side-track of current forcing imbalance, the original topic of this thread and the tables that are the basis of the discussion are the numbers for changes in forcing since 1750, as is customary in this field. That is, incidentally, completely clear from the TAR through AR4, as defined in the glossaries, and in labeling of the various tables.

In that regard you have repeatedly emphasized a 26.5% relative contribution by CO2 to the forcing deltas, in disagreement with IPCC estimates (here, for example), stating that "The IPCC Has Provided An Inaccurate Narrow Perspective Of The Role Of Humans". At this point in the discussion I believe that dana and Tom Curtis have clearly presented why they disagree.

I think it's worth mentioning that while Pielke now says it doesn't matter whether CO2 is responsible for 25 or 50% of the net positive forcing, his own presentations say otherwise.

In his 2006 presentation which we referenced in this post, Dr. Pielke devoted 4 slides to this issue. And on his blog, he has devoted several posts to the subject. And at the Conference on the Earth’s Radiative Energy Budget Related to SORCE on September 20-22, 2006, he made the same argument using the same presentation. The presentation has also been featured on several 'skeptic' blogs (i.e. Jennifer Marohasy and JunkScience). And just last month in an interview with a Canadian newspaper (and in our discussions here), Dr. Pielke argued that too much attention is being paid to CO2 - based on our calculations, that's a hard argument to justify.

Suddenly claiming that the question is an unimportant one seems like a fairly radical and sudden change, given how frequently Dr. Pielke makes this argument. But as KR suggests, we will probably have to agree to disagree and move on.

dana1981 - Your comment and that of the others show why it is futile to debate on this website. You get hung up on one issue where we disagree.

I never focused on the estimate of the fraction of positive radiative forcing (either currently or the change from pre-industrial times) as a primary reason why we need to broaden beyond the radiative effect of CO2. This need is equally true if the fraction is 28% or 50% (or 100% for that matter).

My estimate of the fraction of CO2 was to illustrate with reasonable interpretations from the literature that it may be less than reported in the IPCC report. I came up with ~28%. I adopted a different approach in my response on this weblog post, accepting for the sake of discussion several of your conclusions on the forcing and starting from your fraction and then working with a realistic estimate of black carbon and the two indirect aerosol effects, and the longer period of influence of CO2 to come up with a smaller fraction.

We do not agree on the fraction. In terms of our EOS article and the main issues, this is just a sideshow.

The real substantive issue, however, which no one on this weblog seems to want to debate, is my (and my colleagues) conclusion that

"The IPCC Has Provided An Inaccurate Narrow Perspective Of The Role Of Humans".

Methane: A brief review of the comments did not find anyone who noted the foundation of the Shindell methane estimate of 0.8 W/m2: this is an emissions based estimate, not a concentration based estimate. Shindell's calculations looked at eliminating historical methane emissions (since 1750), resulting in a forcing 0.8 W/m2 lower today: that 0.8 would come partly from an increase in CH4 concentration but also an increase in O3 concentration, stratospheric water vapor, and (this was the novel contribution of the Shindell paper) a reduction in sulfate loading. In contrast, the IPCC estimate is based on concentration of CH4 only. (note that if you use Shindell's estimate, than you can't also use O3 forcing as a separate row - that is definitely double counting).

Black carbon: I will note that the Hansen & Nazarenko estimate of 0.3 W/m2 of snow albedo forcing was obsolete even before it was published, as it was a result of a calculation error (as noted in a later Hansen paper). AR4 estimated 0.1 W/m2, and more recent papers are slightly lower. Of course, since AR4 estimated the direct BC effect at 0.34, for a total of 0.44 (I'd correct the SKS table to reflect that), this isn't a big deal. Though... since BC is rarely emitted without co-emissions of organic carbon and other cooling aerosols, I'm not sure it belongs in this kind of calculation.

Ozone: I don't know why Pielke Sr. thinks that it is a good idea to extrapolate a result for ozone warming in the Arctic in two seasons to annual global forcing.

Albedo: I also don't know why he thinks that a 4 year trend is appropriate to compare to forcing since 1750. I'll note that AR4 estimated -0.2 W/m2 for the long term surface albedo contribution. This probably isn't directly comparable to Pielke's CERES results which are presumably dominated by short-term cloud changes.

Head-of-a-pin: All of this is a bit like counting angels, especially when you begin to throw in aerosols, because of the negative and positive contributions. Is 1.66 W/m2 100% of net forcing? 50% of the SKS list of forcings? 45% if you include black carbon but not BC co-emissions? Whatever. It is pretty clear that CO2 is the single largest contributor to recent and projected future warming, even if it isn't the only contributor globally, and the regional picture gets more complicated with urban heat islands and ENSO variability and so forth.

I don't know if Dr. Pielke has given up on this thread yet, but I would like to just express my understanding of what he means when he says it doesn't make a difference if the anthropogenic forcing is 30% or 50% CO2. In that range, 50-70% of anthropogenic forcing is not CO2, which means those other things should probably account for 50-70% of the discussion about global warming. From my perspective that is certainly a valid argument. I assume that is the point of his argument, but perhaps I'm wrong.

The question in my original post (#21) is related to that. How much of the anthropogenic forcing can be reasonably attributed to fossil fuel extraction/burning regardless of the mechanism for forcing (CO2, methane, aerosol, black carbon, ozone, etc.)? The answer to that question may make the current focus of the public discussion seem more in line with reality.